Yugendo Cave, which possesses strata dating from approximately 350 million years ago, is said to be the oldest limestone cave in Japan. Inside the cave are stalagmites, stalactites, stone pillars, and flowstone, and a mysterious subterranean lake of emerald green. In addition, visitors can view sea lily calyx, which was discovered above bedrock for the first time in Japan here, not to mention other fossils such as brachiopods, fusulinids, and rugose corals, at this veritable natural history museum in a cave.